Anything above 20 kHz will do, oscillations can easily be much higher than that, and at the full voltage swing of the amp (so way beyond rated power).
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Review and Measurements of Purifi 1ET400A Amplifier
- Thread starter amirm
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Not likely according to the article you linked:
I am sure a 25 W amp can damage some speakers under some conditions but I don't agree with the notion that a bigger amp has less chance to do the same, not as a rule anyway. I assume we would likely agree on this point.
In Bruce Morgen's case, it could also be that his speaker's tweeter(s) voice coil happened to have relatively low inductance so they were more prone to the relatively high order harmonics resulted from clipping as that could result in excessive heat/thermal failure if the high order harmonic contents were high enough.
@Bruce Morgen, did the tweeter(s) fail quickly, or after a long listening session?
@AdamG247 @BDWoody Should the clipping discussion be moved to a new thread? Not really anything to do with PuriFi per se...
Much older clipping article: https://www.audiosciencereview.com/forum/index.php?threads/clipping-101.7164/
Much older clipping article: https://www.audiosciencereview.com/forum/index.php?threads/clipping-101.7164/
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Indeed. I think that article addresses the misbeliefs about clipping very well, so hopefully we can move on from an isolated incident that happened 45 years ago...
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so hopefully we can move on from an isolated incident that happened 45 years ago...
So "isolated" that a major speaker mfr. had to design and offer an accessory literally called the "Tweeter Saver" -- because so many low power 1970s amplifiers went into wildly destructive, inaudibly high frequency oscillation when pushed past their rated output.
I think the article by @DonH56 is a good read, and I have nothing to add to what already has been written by him, me, Phil Allison and Rod Elliott. Let's get back to discussing the Purifi amp.So "isolated" that a major speaker mfr. had to design and offer an accessory literally called the "Tweeter Saver" -- because so many low power 1970s amplifiers went into wildly destructive, inaudibly high frequency oscillation when pushed past their rated output.
restorer-john
Grand Contributor
Don's article is about clipping. It doesn't address the biggest issue with clipping, that it introduces instability (due to the amplifier temporarily running open loop) and oscillation. And it is the HF oscillation that kills the tweeters.
Bruce is right. For those of us who were around selling gear in the early digital days when amplifiers didn't know what had hit them, there were a number of loudspeaker manufacturers who had to come up with tweeter protection really fast or be stuck with an insane number of dubious warranty claims.
And, now, the issue is far from 'solved'. People with low powered amps, be they class 'whatever' and expecting higher levels than the gear can achieve.
Sure, we can hive off a clipping thread, but clipping issues are applicable and can be discussed about any amplifier.
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restorer-john
Grand Contributor
I've seen (and had) tweeters expire in an instant from high powered amplifiers that clipped and oscillated.
Indeed. I was really addressing the "(Clipping) low power amps kill speakers, higher powered amps don't" myth. I agree the issue is more likely oscillation. This is relevant to the Purifi discussion in that one of the main things Bruno Putzeys has brought to the class D game is the thorough analysis and understanding of feedback and stability in class D amps.
@restorer-john brings up a good point, though again this discussion is way off-topic IMO. I did not consider amplifier (in)stability in my clipping article because I consider it a separate issue. High-frequency oscillation will almost certainly destroy a tweeter, clipped or not, almost instantly. Things like "motor-boating" and other lower-frequency instabilities can blow out other drivers. Heck, a full-power turn-on surge ("pop") can do some damage!
You may not have to overdrive an amplifier to have it oscillate; the wrong load (speaker impedance) can do it. It is also worth noting that clipping is not always as clean as my article indicates; some amplifiers will "fold back" or generate otherwise very ugly asymmetric waveforms (@amirm has shown examples before), and some will break into oscillation when clipped. My article only addresses a well-behaved amplifier that is overdriven resulting in a clipped output. Alas, not all amplifiers are so well behaved.
As @Julf said, part of the work Bruno et. al. did was to develop a thorough understanding of feedback and how it is applied, and misapplied, to make amps like the Purifi models well-behaved.
IME/IME/etc. - Don
You may not have to overdrive an amplifier to have it oscillate; the wrong load (speaker impedance) can do it. It is also worth noting that clipping is not always as clean as my article indicates; some amplifiers will "fold back" or generate otherwise very ugly asymmetric waveforms (@amirm has shown examples before), and some will break into oscillation when clipped. My article only addresses a well-behaved amplifier that is overdriven resulting in a clipped output. Alas, not all amplifiers are so well behaved.
As @Julf said, part of the work Bruno et. al. did was to develop a thorough understanding of feedback and how it is applied, and misapplied, to make amps like the Purifi models well-behaved.
IME/IME/etc. - Don
mk1classic
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How can we retrace this discussion back to be relevant to the Purifi 1ET400A amplifier?
Do we see a possible problem with HF oscilation with the 1ET400A design? We have seen hf measurements but not under high load?
Do we see a possible problem with HF oscilation with the 1ET400A design? We have seen hf measurements but not under high load?
Pretty impossible.
No, HF oscillation is definitely not an issue with the 1ET400A (or the earlier Hypex ones for that matter).
Pretty impossible.
No, HF oscillation is definitely not an issue with the 1ET400A (or the earlier Hypex ones for that matter).
I hope not as I have one each. Such oscillation issue under specific load conditions might have been more common in the old days, I would hope that the newer amps should be virtually immune to that in most cases. I would love to read even one review with measurements such as Stereophile and ASR's that encounter such as issue that caused trouble (including failure) with the amp under test, let alone speakers.
I only see the PWM driver...WHERE are the FINAL stage mosfets...(are those weird looking vertical boards chips) ?Have to say the Purifi 1ET400A sounds fabulous very crudely set up on the test bench here
View attachment 29833
PS : is the isolated transformer for FEEDBACK...? i know so many questions...The Patent talks about lotsa feedback from output (Negative) compared to input to keep the Load in Check...
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Mosfet are on the bottom side of the board. The transformer is the output filter.
I have a suggestion but I may be partial.
TabCam
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There is not that much between the various versions. If you want a multichannel version you could also look at the Apollo Audio offering which has the ET7040SA version beside the regular ET400A also as an option.
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